1. Field of the Invention
The present invention relates to an optical pickup for recording or reproducing information optically in recording media such as plural kinds of optical discs, and an optical disc device having such optical pickup.
2. Description of the Related Art
Recently, as blue-violet semiconductor lasers are put in practical use, a Blu-ray disc (BD) is practiced as an optical recording medium (also known as optical disc) of high density and large capacity realized in a same size as compact disc (CD) or digital versatile disc (DVD). The BD is an optical disc of about 0.1 mm in thickness of protective substrate capable of recording or reproducing with a blue-violet laser beam source of about 400 nm in wavelength, and an objective lens enhanced in numerical aperture (NA) to 0.85.
On the other hand, there is also realized an HD DVD of 0.6 mm in thickness of protective substrate, using a blue-violet laser beam source of about 400 nm in wavelength, and an objective lens of numerical aperture of 0.65.
A compatible optical pickup is proposed for recording or reproducing information by focusing laser beam with one objective lens on the information recording surface of optical discs that is different in the thickness of protective substrate.
An optical pickup having a focusing optical system capable of focusing laser beam up to diffraction limit on optical discs differing in thickness of protective substrate is disclosed in Japanese Unexamined Patent Publication No. 7-98431 and 10-10308.
A configuration of a conventional optical pickup disclosed in Japanese Unexamined Patent Publication No. 7-98431 is shown in FIG. 18. In FIG. 18, an optical pickup 130 includes a first light source for emitting a red laser beam 101, a beam splitter 103, a collimator lens 104, a diffractive lens 105, an objective lens 106, a detection lens 108, and a light detector 109. Reference numeral 70 is a DVD, which is an optical disc of 0.6 mm in thickness of protective substrate.
Operation of optical pickup 130 for recording or reproducing information in a first low-density disc (DVD) 70 is explained. The red laser beam emitted from the first light source 101 passes through the beam splitter 103, and is converted into substantially parallel light by the collimator lens 104, and further passes through the diffractive lens 105, and is focused by the objective lens 106 into a light spot on the information recording surface of the first low-density disc 70 over the protective substrate. The red laser beam of the backward path reflected by the information recording surface of the first low-density disc 70 passes through the objective lens 106, diffractive lens 105, and collimator lens 104 via the same optical path as the forward path, and is reflected by the beam splitter 103, and is provided with a specified astigmatism by the detection lens 108, and is guided into the light detector 109, and the light detector 109 generates information signal and servo signal.
Referring now to FIG. 19, operation of optical pickup 130 is explained about recording or reproducing on a second low-density disc 80 or an optical disc of 1.2 mm in protective substrate thickness. The red laser beam emitted from the first light source 101 passes through the beam splitter 103, and is converted into substantially parallel light in the collimator lens 104, and is further diffracted in the diffractive lens 105, and is focused by the objective lens 106 into a light spot on the information recording surface of the second low-density disc 80 over the protective substrate. The red laser beam of the backward path reflected by the information recording surface of the second low-density disc 80 passes through the objective lens 106, hologram 105, and collimator lens 104 via the same optical path as the forward path, and is reflected by the beam splitter 103, and is provided with a specified astigmatism by the detection lens 108, and is guided into the light detector 109, and the light detector 109 generates information signal and servo signal.
As for the focus error signal for recording or reproducing the first low-density disc 70 or second low-density disc 80, a method of astigmatism may be applied, that is, the focusing spot provided with astigmatism by the detection lens 108 is detected by four-division pattern in the light detector 109. As for the tracking error signal a so-called three-beam method or differential push-pull (DPP) method with main beam and sub-beam produced by diffraction grating (not shown) may be applied.
Function of diffractive lens 105 and objective lens 106 is described specifically by referring to FIG. 20, FIG. 21A, and FIG. 21B.
The diffractive lens 105 is provided with a diffraction pattern 105a as shown in FIG. 20 in order to focus tiny light spots in the first low-density disc 70 and second low-density disc 80. The diffractive lens 105 has a diffraction efficiency of +primary diffraction light of less than 100%, and is designed to have a sufficient intensity of transmission light (also known as 0-order diffraction light, and transmission light is one of diffraction light). By blazing the diffraction light 105, the sum of quantities of light of 0-order diffraction light and +primary diffraction light can be increased, and the use efficiency of light is enhanced.
The objective lens 106 has the numerical aperture NA of 0.6, and is designed to form a focusing light spot of diffraction limit on the first low-density disc 70 of 0.6 mm in protective substrate thickness when receiving laser beam directly without diffraction by the diffractive lens 105 (that is, 0-order diffraction light) as shown in FIG. 21A.
On the other hand, as shown in FIG. 21B, +primary diffraction light diffracted by the diffractive lens 105 is focused on the second low-density disc 80 by the objective lens 106. Herein, the +primary diffraction light is corrected in aberration so as to form a light spot of diffraction limit on the second low-density disc 80 of 1.2 mm in protective substrate thickness.
Thus, by combining the diffractive lens 105 and objective lens 106 for diffracting part of incident light, a two-focus lens capable of forming focusing spots focused to diffraction limit on optical discs mutually different in substrate thickness is realized.
Since the diffractive lens 105 has a lens action, positions of two foci in an optical axis direction are different. Therefore, while recording or reproducing information with the light spot formed in one focus, the light spot formed in other focus is spread widely, and no effect is given to recording or reproducing of information.
As described above, the use of the optical pickup 130 can record or reproduce the information with one objective lens on different kids of optical disks.
Japanese Unexamined Patent Publication No. 2006-147075 discloses an optical pickup applicable to four kinds of optical discs, that is, CD, DVD, BD, HD DVD, by changing over the combination of two objective lenses and light sources. Japanese Unexamined Patent Publication No. 2001-344803 discloses a configuration for corresponding to plural kinds of optical discs by optically changing over the optical path of the laser beam emitted from one light source.